An “analyzer” in the sense of this invention shall mean a measuring apparatus in process automation engineering that measures with a wet-chemical method certain substance contents, such as, for example, the ion concentration in a medium that is to be analyzed. For that purpose, a sample is taken from the medium that is to be analyzed. Usually, the taking of the sample is performed by the analyzer itself in a fully automated fashion by means such as pumps, hoses, valves etc. For determining the substance content of a certain species, reagents that have been developed specifically for the respective substance content and that are available in the housing of the analyzer are mixed with the sample that is to be measured. A color reaction of the mixture caused in this way is subsequently measured by an appropriate measuring device, such as, for example, a photometer. To be more precise, sample and reagents are mixed in a cuvette and then optically measured with different wavelengths using the transmitted light method. Thus, the measured value is determined by the receiver based on light absorption and a stored calibration model.
In this context, it is extremely important to know the exact amount of different liquids that are being mixed with each other. For a correct determination of the substance content, it is required to precisely define the amount of the sample to be measured as well as the amount of the reagents to be mixed into it.
One possibility of measuring a certain amount of a liquid consists in filling a transparent tube 1 that has a known volume up to a certain defined fill level. It can be optically detected, when the desired fill level is reached. This is done by utilizing the fact that a filled tube possesses different refractive properties than an empty one. To do so, a light barrier with a light source and a light detector that are arranged on a tube are used. Depending on the arrangement, the received amount of light increases or decreases with the filled tube compared to the empty tube. In FIG. 1, the situation that is created with centered radiation is shown. FIG. 2 shows the conditions with eccentric radiation. In any case, the received amount of light changes depending on the tube being empty or filled. Here, setting a precise threshold value poses a problem, i.e. for determining whether the required amount of liquid has been filled in, because the refractive properties of the filled tube depend on the refractive index of the filled-in liquid. Moreover, turbidity of the liquid, dirt on the tube and deviations from the geometric ideal shape influence the value. In case of tubes with a small diameter, inaccuracies (manufacturing tolerances) in the arrangement of the components also play an important role.